Never too early to prepare for the future. Fixed based on this recommendation
django.utils.deprecation.RemovedInDjango20Warning: on_delete will be a required arg for
ForeignKey in Django 2.0. Set it to models.CASCADE on models and in existing migrations if you
want to maintain the current default behavior.
This was causing open-craft/openedx-completion-aggregator#4 tests to fail once I added completion as a depenency, even on django 1.11. Probably some configuration was causing deprecation warnings to get promoted to errors. :woman_shrugging:
Review: @ciuin, @iloveagent57